I remember the series being called something like "Ghost World" or "Ghost" something. The issue is that there is already a story called Ghost World. I've done quite a bit of searching. It does quite appear that perhaps the series never went online or my Search-fu is weak.
The story starts with a female ghost pulling a live boy into her world that she is allowed to leave once a year visiting a house where she lived / died. I didn't read the first book, so I'm going off of callbacks.
The ghost world is quite fantastical and would remind you of The Seventh Tower. The story revolves around the girl and the boy attempting to find a way back to the real world while having many misadventures. The ghost girl eventually falls for the boy.
It has been several decades and I'm curious as to whether the boy ever did escape or what the love interest angle would be. Did he decide to stay after all? I doubt it.
If I remember correctly, the series was by two authors and it was released in the '90s by scholastic.